This application claims priority from Indian patent application 609/CHE/2006, filed on Apr. 3, 2006. The entire content of the aforementioned application is incorporated herein by reference.
With the number of users of Voice over IP (VoIP) systems on the increase, many service providers wish to give a warranty for the service they provide concerning transmission and fault performances to their customers. Furthermore, there is a need to derive a quantitative measure of the quality of the service delivered, to monitor the service and to investigate root causes of any faults occurring in a network. This will enable SLAs (service level agreements) for customers to be shaped, monitored and their violations compensated. The reason for doing this is to mitigate major market inhibitors for most VoIP deployments.
VoIP is a telephony application where the IP network facilitates packet data transport for discrete encoded samples of the voice, as well as signaling information exchanged between entities involved in this service. In order to obtain a good audio quality, it imposes strict real-time and network performance requirements that must be met by the IP network as closely as possible. This is especially important for the sessions set up for transmission of voice.
In the Internet world, however, since services are based on a network service model (=the services realized on the network layer), which is also referred to as “best effort model”, no guarantees are provided concerning quality-of-service of transmission of data streams. The Internet protocol neither provides a bandwidth guarantee nor does it guarantee that the transmission is without any loss. Furthermore, data packets may arrive in any sequence at the destination, there is no guaranteed transmission time and no indication about congestion. In this respect, the Internet network service model differs, for example, from service network models provided in ATM (asynchronous transfer mode) networks. One network service model is, for example, a CBR service model. CBR (constant bite rate) enables a constant transmission rate and a loss-less transmission rate. The sequence of data packets is preserved and congestion does not occur. Further service network models are “variable bit rate (VBR)”, “available bit rate (ABR)” and “unspecified bit rate (UBR)” which provide different quality-of-service which all provide higher quality than IP's “best effort” service model.
The evolution of Internet and ATM network service models also reflects their origins. With the concept of virtual channels (VCs) as a central organization principle, ATM cannot deny its origins from telephony (in which “real circuits” are used). A network based on virtual circuits is arguably more complex than a datagram network. (A datagram is a self-contained packet, one which contains enough information in the header to allow the network to forward it to the destination independently of previous or future datagrams. The terms “datagram” and “packet” are synonymously used herein.) In a datagram network, datagrams only carry their source and destination address and are sent from one router to another, whereby each router knows how to forward the datagrams. Telephone networks, by necessity, had their complexity within the network, since they were connecting dumb end-system devices such as rotary telephones.
The Internet as a datagram network, on the other hand, grew out of the need to connect computers together. Given more sophisticated end-system devices, the Internet architects chose to make the network-layer service model as simple as possible. Additional functionality, for example, in-order delivery, reliable data transfer, congestion control, and DNS name resolution is then implemented at a higher layer, in the end systems.
With regard to VoIP projects, this means that mechanisms for monitoring quality-of-service are established in order to enable a VoIP provider and/or a network provider to give a warranty for the service she/he provides to a client.
A warranty for a service is usually contractually agreed upon by means of an SLA. The TeleManagement Forum's SLA Management Handbook defines an SLA as “[a] formal negotiated agreement between two parties, sometimes called a service level guarantee. Typically, it is a contract (or part of one) that exists between the service provider and the customer, designed to create a common understanding about services, priorities, responsibilities, etc.”
Historically, service level agreements arose in the early 1990s as a way of measuring and managing quality-of-service (QoS) that IT departments and service providers within private (usually corporate) computer networks delivered to their internal customers. To this end, network management tools have been developed, whereby many of the tools rely on the simple network management protocol (SNMP) which is part of the Internet protocol suite and is located on the application layer. SNMP is a client-server based management protocol in which a client queries variables which are stored in local databases, commonly referred to as management information bases (MIB), of the components to be monitored. It should be mentioned that the MIBs are at least partly configured by the manufacturer of the device; some manufacturers do not support MIBs.